1. Field of the Invention
The present invention relates to a method and an apparatus for controlling transmission power in a communication terminal.
2. Description of the Related Art
In general, an important factor for improving signal transmitting/receiving performance is the control of transmission power in a communication system. In the case of an uplink, a Mobile Station (MS) transmits a signal by minimizing power consumption in a range where a Base Station (BS) can normally receive the signal from the MS.
Hereinafter, it is assumed that a communication system uses an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. Also, a communication system that uses the OFDMA method is referred to as an “OFDMA communication system.”
In the OFDMA communication system, an MS transmits an uplink signal to a BS using transmission power determined by a preset power control algorithm. The BS receives the uplink signal, and estimates the Carrier to Interference and Noise Ratio (CINR). The BS compares the estimated CINR with a preset reference CINR, and transmits a control message for controlling the transmission power of the MS in accordance with the result of the comparison to the MS. Here, the control message includes transmission power control information for controlling the transmission power of the MS. The MS receives the control message transmitted from the BS, determines the transmission power using the transmission power control information included in the received control message, and transmits a subsequent link signal using the determined transmission power.
In determining the transmission power, the MS uses a normalized Carrier to Noise ratio (C/N) as represented in Table 1 below.
TABLE 1Modulation/FEC rateNormalized C/NACK region−3.0FAST FEEDBACK0CDMA code3QPSK ⅓0.5QPSK ½6QPSK ⅔7.5QPSK ¾916-QAM-½1216-QAM-⅔14.516-QAM-¾1516-QAM-⅚17.564-QAM-½1864-QAM-⅔2064-QAM-¾2164-QAM-⅚23
In Table 1, normalized C/Ns corresponding to modulation/Forward Error Correction (FEC) rates are shown. As can be seen in Table 1, as the level of modulation/FEC is increased, the corresponding normalized C/N increases. That is, among the modulation/FEC rates, the modulation/FEC rate corresponding to the ACKnowledgement (ACK) region has the lowest level, and the modulation/FEC rate corresponding to the 64-Quadrature Amplitude Modulation (QAM)-5/6 has the highest level.
As shown in Table 1, in the case of transmitting an ACK region signal, the MS uses transmission power of −3.0 dB by default, while in the case of transmitting a Quadrature Phase Shifting Keying (QPSK)-1/2 signal, the MS uses transmission power of 6 dB by default. Here, the QPSK is a kind of modulation method, and 1/2 indicates a code rate. FIG. 1 is a flowchart illustrating an example of the process of transmitting/receiving signals using a normalized C/N table in an OFDMA communication system according to the related art. Here, it is assumed that Table 1 is used as the normalized C/N table.
Before describing the process as illustrated in FIG. 1, in the case of using the normalized C/N corresponding to the modulation/FEC rate as a default value as indicated in Table 1, the receiving performance for each modulation/FEC rate may differ depending on the channel state between the BS and the MS. Accordingly, the CINR required for the respective MS may be differently set depending on the channel state of the MS. Here, the required CINR denotes the CINR required for the BS to normally receive the signal of the MS.
Table 2 below represents Bandwidth Request (BR) Code Division Multiple Access (CDMA) code required CINRs and QPSK-1/2 required CINRs.
TABLE 2BR CDMA CodeQPSK ½Required CINRRequired CINRChannel A2 dB4 dBChannel B2.5 dB  5 dBChannel C3 dB6 dB
Referring to Table 2, it is assumed that MS A using channel A, MS B using channel B, and MS C using channel C exist, and that a channel between MS A and the BS, a channel between MS B and the BS, and a channel between MS C and the BS are called “channel A”, “channel B”, and “channel C”, respectively. It is also assumed that channel A is best, channel B is worse than channel A but better than channel C, and channel C is good. In this case, MS A is set to have the lowest BR CDMA code and the lowest QPSK-1/2 required CINR, and MS C is set to have the highest BR CDMA code and the highest QPSK-1/2 required CINR. Accordingly, it is required that the BS sets the normalized C/N value based on the channel having the worst channel state, i.e. channel C. That is, as represented in Table 1, the normalized C/N is determined by reflecting the requested CINR of the MS having the worst channel state.
Referring to FIG. 1, it is assumed that the channel state between MS 100 and BS 150 corresponds to channel A. In step 111, in the case of transmitting QPSK-1/2 data, MS 100 sets the default value of the transmission power to the normalized C/N of 6 dB as represented in Table 1. MS 100 transmits the QPSK-1/2 data to BS 150 using the determined normalized C/N of 6 dB. In step 115, BS 150 determines the QPSK-1/2 required CINR of 4 dB and the MS required CINR of 4 dB through a power control algorithm. Accordingly, although not illustrated in FIG. 1, BS 150 informs MS 100 that the required CINR is 4 dB.
If BS 150 informs MS 100 that the required CINR is 4 dB, MS 100 sets its own set point to 4 dB. BS 150 transmits a power control message including the corresponding transmission power correction information to MS 100. MS 100, having received the message, determines the transmission power that suits the required CINR of 4 dB through the transmission power correction information. If it is required for MS 100 to transmit the BR CDMA code in step 120, MS 100 detects that the modulation/FEC rate level of the BR CDMA code is lower than the modulation/FEC rate level of the QPSK-1/2 data with reference to Table 1. Also, MS 100 detects that the normalized C/N of the BR CDMA code is 3 dB with reference to Table 1, and detects the difference between the detected normalized C/N and the normalized C/N applied to the previously transmitted signal, i.e. 6 dB applied to the QPSK-1/2 data, which is 3 dB. Then, since the modulation/FEC rate level of the BR CDMA code to be currently transmitted is lower than the modulation/FEC rate level of the previously transmitted QPSK-1/2 data, MS 100 determines the transmission power by reflecting the difference value of 3 dB between the previously transmitted power value, i.e., the power with which the QPSk-1/2 data was transmitted, which is 6 dB, and the normalized C/N of the BR CDMA code, which is 3 dB, and transmits the BR CDMA code to BS 150.
In step 125, Since the received CINR of the BR CDMA code, which is 1 dB, is less than the required CINR of Table 2, which is 2 dB, the probability that BS 150 cannot receive the BR CDMA code of MS 100 is increased.
The reason why BS 150 cannot receive the BR CDMA code is that MS 100 determines the transmission power of the BR CDMA code in consideration of only the CDMA code normalized C/N of 3 dB and the QPSK-1/2 normalized C/N of 6 dB in Table 1. That is, although the set point of MS 100 has already been set in consideration of the required CINR received from BS 150, MS 100 unconditionally determines the transmission power of the BR CDMA code in consideration of only the normalized C/N difference in Table 1, and thus BS 150 cannot receive the BR CDMA code.
As described above, the reason why the BS cannot receive the BR CDMA code in FIG. 1 is that the MS uses only one fixed normalized C/N table without considering wireless channel circumstances.
FIG. 2 is a flowchart illustrating another example of a process of transmitting/receiving signals using a normalized C/N table in an OFDMA communication system according to the related art. Here, it is assumed that Table 1 is used as the normalized C/N table.
Before describing the process as illustrated in FIG. 2, it is assumed that, unlike Table 1, the normalized C/N for use in the QPSK-1/2 data is 5 dB, and the channel state between MS 200 and BS 250 corresponds to channel C.
Referring to FIG. 2, in the case of transmitting the BR CDMA code in step 211, MS 200 transmits the default value of the transmission power to BS 250 using the transmission power of the normalized C/N of 3 dB as represented in Table 1. In accordance with the power control algorithm, in step 215, the BR CDMA required CINR is set to 3 dB, and although not separately illustrated in FIG. 2, BS 250 transmits a power message including the corresponding transmission power correction information to MS 200, and MS 200, having received the power message, determines the transmission power that suits the required CINR of 3 dB by reflecting the transmission power correction information.
In step 220, BS 250 transmits a CDMA allocation Information Element (hereinafter referred to as “CDMA Alloc IE”) including bandwidth allocation information in accordance with a bandwidth allocation request of MS 200 to MS 200.
MS 200, having received the CDMA Alloc IE, detects that it should transmit a BR header. Here, since the BR header is the QPSK-1/2 data, MS 200 detects that the modulation/FEC rate level of the QPSK-1/2 data is higher than the modulation/FEC rate level of the CDMA code with reference to Table 1. Also, MS 200 detects that the normalized C/N of the QPSK-1/2 data is 5 dB and detects the difference between the detected normalized C/N and the normalized C/N used in the previously transmitted signal, i.e. the BR CDMA code, which is 2 dB. Then, since the modulation/FEC rate level of the BR header to be currently transmitted is higher than the modulation/FEC rate level of the previously transmitted BR CDMA code, MS 200 determines the transmission power by reflecting the difference value of 2 dB between the normalized C/N of the QPSK-1/2 data, which is 5 dB, and the previously transmitted power value, i.e., the power with which the BR CDMA code was transmitted, which is 3 dB, and then in step 225, transmits the BR header to BS 250 with the determined transmission power. However, since the received CINR of the BR header, which is 5 dB, is less than the required CINR represented in Table 2, which is 6 dB, the probability that BS 250 cannot receive the BR header is increased in step 230.
As described above with reference to FIGS. 1 and 2, since the MS in the OFDMA communication system according to the related art transmits signals using the preset normalized C/N irrespective of the channel state, the BS may not receive the signal from the MS, and this may cause the performance of the OFDMA communication system to deteriorate.
Therefore, there exists a need for an improved apparatus and method for controlling transmission power in a communication terminal.